This invention relates to the fields of lasers and optics.
A frequent problem in the performance of solid state lasers is optical damage to the dielectric coatings forming the output coupler. Such multilayer dielectric-film coatings are generally the weakest element in a laser system, and typically fail at intensities below 10 GW/cm.sup.2 or fluences below 5 J/cm.sup.2. In high-gain pulsed lasers, the optical intensity at the output coupler is often larger than at other surfaces, making the output coupler a common source of problems.
In contrast to dielectric films, there are many bulk optical materials with a damage threshold in excess of 100 GW/cm.sup.2. As a result, polished etalons made from dielectric materials, such as quartz or sapphire, with highly parallel faces are often used as the output mirrors for pulsed high-power lasers. That is, the lasers are operated with a 100% mirror on one end and a polished etalon a few millimeters or a centimeter thick, generally with no additional coatings, as the output coupler on the other end. Since these lasers typically have large round-trip gains, they operate best with low-reflectivity output mirrors, and the uncoated dielectric etalon provides a simple way of achieving the necessary output mirror reflectivity. These uncoated etalons are simple to fabricate and can have very high optical-damage thresholds.
If a bulk etalon, as described above, is used in a solid-state laser, at least one end--the output end--of the solid-state gain medium must be treated to eliminate reflections at the solid-to-air interface. This could be done by depositing a dielectric antireflection coating on the gain medium, or by cutting the gain medium at Brewster's angle. The use of such a dielectric coating can result in a lower threshold for optical damage. Cutting the gain medium at Brewster's angle complicates the fabrication of the device and can lead to poorer performance.